Control system



Jan. 31, 1950 A. J. HORNFECK CONTROL SYSTEM '1 Sheets-Sheetl Filed June 6, 1946 @EHIP mwawa INVENTOR.

ANTHONY J. HORNFECK w. mwIjaEq ATTOR E Y Jan. 31, 1950 A. J. HORNFECK 2,495,344

CONTROL SYSTEM Filed June 6, 1 946 7 Sheets-Sheet 2 ONE CYCLE 0E CAM 6| IN-PHASE OUT-QF-PHASE VOLTAGE ENVELOPE Sp SET AT 50% Sb SET AT MAX.

FIG. 2

"OFFHTIME +-"ON"T|ME OF HEATER l3 lN-PHASE OUT- -'-PHAsE 1m 'ENIOR.

APHHCWY d. HORNF'EC-hl FIG. 3 4 O Jan. 31, 1950 Filed June 6, 1946 A. J. HORNFECK CONTROL SYSTEM 85 49A I 49A 49B 5- 498 r 93 as 1 95 21% 7 Sheets-Sheet 3 INVENTOR.

ANTHONY J. HORNFECK Jan. 31, 1950 Filed June 6, 1946 A. J. HORNFECK CONTROL SYSTEM 7 Sheets-Sheet 4 ELECTRONIC RELAY 26A I LIJ IINVENTOR.

ANTHONY J. HORNFECK Jan. 31, 1950 A. J. HORNFECK 2,495,844

CONTROL SY$TEM Filed June 6, 1946 '7 Sheets-Sheet 5 MODULATOR BRIDGE fig I33 I I36 I35 RESET :50 7 RS RM. 5 N E INVENTOR.

ANTHONY J. HORNFECK Jan. 31, 1950 A. JLH RNFEcK 2,495,344

CONTROL SYSTEM Filed June 6, 1946 7 Sheets-Sheet 6 53 RA Wm gI36 RATE OF RATE OF MAX. SINGLE POINT INCREASE DECREASE VALUE VALUE I3! \29 \30 36 SINGLE 1 po T, PROGRAM CONTROL 26 n CONTROL \25 I28 RESET INVENTOR. F 5 ANTHONY J. HORNFECK 7" ZZW Jan. 31, 1950 J, HORNFECK 2,495,844

CON'I'ROL'SYSTEM Filed June 6 1946 v 'T Sheets-Sheet '7 MODULATOR TO ADDlTiONAL CONTROLLERS I FIG. I3

INVENTOR.

ANTHONY J. HORNFECK MOTOR OF MOTOR OF BY IG. 5 FIG. 9 No/Q FIG. '2 A7TOREY Patented J. 31, 1950 CONTROL SYSTEM Anthony J. Hornfeck, Cleveland Heights, Ohio,

aasignor to Bailey Meter Company, a corporation of Delaware Application June 6, 1946, Serial No. 674,752

13 Claims.

My invention relates to electrical control circuits and particularly to electrical control circuits utilizing a measurement of a variable quantity, quality, position, or the like as the motivating basis for a control of the same or of another variable. A condition, quantity, quality, position or other variable which may be represented by an electrical value, such as rmistance, potential or other electrical characteristic may be continuously and instantaneously measured through the agency of the circuit to be described. The measurement so obtained may be used to effect a control of the same or of another variable which may or may not contribute to the magnitude or change in magnitude of the variable being measured.

Representative of variable quantities, conditions and the like to which my invention is directed are such variables as rate of fluid flow, temperature, liquid level, pressure; although the variable may equally as well be the position in space of an object, the throttling position of a valve, or the like.

In the control of combustion or of other processes the time of response (process lag) of the system to a corrective change in the rate of application of an agent, following a departure of the controlled condition from its desired value, depends on various constructional and operating factors. Overtravel, hunting, or the like of a control system results in inefllciency, waste and excessive wear on equipment. To satisfactorily control the operation of different processes and apparatus several basic types ofcontrol are known and are widely used in pneumatic and hydraulic control systems. A principal object of my present invention is the embodiment in electrical and electronic circuits of the proven desirable features of control readily adaptable to a wide variety of process systems to be controlled.

By way of example only I have chosen to illustrate and describe my invention as applied to an electrically heated furnace wherein it is desired to maintain a uniform preselected temperature. It will be understood that such example is representative of a variable condition to be controlled through the application of an agent 7 such as a fuel supply valve, for example, is in either open or shut position; or an electric heating-resistor is turned on or off by a contactor.

In modulating control the valve is positioned between its extremes of travel to modulate or throttle the rate of flow of fuel through the valve. A modulating control while preferable is usually a more elaborate and expensive type and is not readily applicable to controlled elements which have only two possible positions, such as a solenoid operated valve which has only an open and a shut position, or an electric heating resistor element to which the current is either on or off. Such on-ofi" control is however at one or the other of two extremes and may lead to bad cycling.

The dead band of the controller is that range in temperature across the standard to which the controller is insensitive. In other words, a portion of the range of the instrument through which temperature may vary without effecting an actuation of the control. For example, assume that the range of the measuring controller is 0 to 1000" F. and'the control point or desired standard temperature is 500 F. A desirable dead band might for example be the temperature between 499 F. and 501 F., a deadband of 2 F. Usually the possibility exists of affecting the condition and is not to be taken as two general types of control are recognized which are referred to as "on-oil and modulating." In on-oil control the controlled element,

frequently when temperature remains almost exmoving the dead band" without change along with the control point. This means that a dead band of 2 F. would exist across a control point of 450 F., 500 F., 700 F. or substantially any selected standard temperature in the given range of 0 to 1000 F. Under certain conditions of apparatus or circuit arrangement the dead band may vary slightly in width at different points in the measuring range. For the purpose of explanation, however, I assume'that the dead band will remain substantially uniform across whatever control point is selected in the range of the instrument. v

It will be appreciated that, if the dead band is substantially non-existent, then the slightest variation in temperature in one direction or the other from the control point of 500 F. will result in an actuation of the controlled element and an undesirable cycling may occur with the resistor being turned on and off rapidly or too actly on the desired value.

If the dead band is too wide, then temperature will not be held closely, for it will require a away that when a change in the fuel supplyrate is made over-regulation and hunting occur. The actual width of the dead band selected will depend, of course, upon the characteristics of the system in relation to how close the temperature of the furnace must be maintained.

So long as temperature is above standard, current to the furnace heating resistors is turned "off." If the temperature is at or below standard the current is turned on. Obviously, the full heating effect of the resistor may be greater than is necessary to balance heat input to heat output for maintaining a uniform desired temperature. Preferably the amount of correction to rate of heat input should bear a relation to the extent of departure of temperature from standard. Modulating or throttling control tends to maintain the application of the corrective agent in accordance with the demand.

It is a particular object. of my present invention to provide a controller for an on-oi!" type of controllable apparatus embodying or incorporating the advantageous features of a modulating" type of control. Basically, this result is accomplished by varying the proportion of "ontime" to the off-time," giving the effect of a modulation related to the extent of departure of temperature from standard, and therefore to the actual value of temperature in relation to the standard value.

In the drawings:

3 Fig. 1 is a measuring and control system in connection with a resistance thermometer.

Figs. 2 and 3 are graphs of electrical characteristics in connection with Fig. 1.

Fig. 4 illustrates a modification of a portion 35 only of Fig. 1

Fig. 5 is a modification of a portion of Fig. 1.

Fig. 6 is a graph of operation in connection with the arrangement of Fig. 5.

Figs. 7 and 8 diagrammatically illustrate modifications of the circuit shown in Fig. 5.

Fig. 9 is a circuit diagram of a modification of a part of Fig. 1-

Fig. 10 is a simplified across-the-line diagram of Fig. 9.

4 Fig. 11 is a suggested control panel board arrangement in connection with Figs. 9 and 10.

Fig. 12 is a diagram comparing the operation of the motors of Figs. 5 and 9.

Fig. 13 illustrates how the circuit of Fig. 1 may 50 be pluralized.

All of the figures of the drawing are shown in quite diagrammatic or schematic manner. In Fig. 1 I have shown the amplifier and motor con- The electric furnace 6 is preferably heated by an electric resistance element l3 receiving current from an A.-C. source II, which also provides *alternating current supplied to the bridge i 5 through a transformer ii.

Preferably the bridge arm 5 is a platinum resistance measuring element. The conjugate corners of the bridge I are connected to an amplifier I6 and motor control I! for the motor 8. For an 10 understanding of the phase sensitive A.-C. bridge,

for measuring the resistance of the leg 5 sub- Jected to temperature of the furnace 6, reference may be had to the Ryder Patents 2,275,317 and 2,333,393. The voltage output of the bridge I,

supplied to the amplifier i6, assumes a balance or unbalance and a phase relation relative to the supply voltage dependent upon the magnitude and sense of the unbalance condition of the bridge. The amplifier l6 selectively controls motor tubes l8, I9, which in turn control the amount and direction of unbalance of saturable core reactors and 2i for directional and speed control of the motor 9 adapted to position the arms 8 and iii.

The motor 9 is of an alternating current type having windings 22 and 23 ninety electrical degrees apart and also having a capacitor 24. When alternating current passes directly through one of the windings and simultaneously through the other winding in series with the capacitor the motor rotates in predetermined direction and at a speed determined by the extent of unbalance of the saturable core reactors 20, 2!. It is not necessary to go into greater detail as to the construction and operation of the amplifier I6 and the motor control circuit H, as reference may be had to the above mentioned Ryder patents.

In my present invention, in addition to providing an instantaneous indication and a continuous record of the value of temperature to which the arm 5 is subjected, I provide an electrical control of the heat input of the resistor It to the electric furnace 6. I have shown in this connection a power controller 25 which forms no part of the present invention. This may be any adaptable power controller for the resistance i3. My present invention resides in an electrical system intermediate the measuring system and the power controller, sensitive to the measuring system, and for turning on and oflf the power controller 25. Thus, upon departure of temperature within the furnace 6 from that which is desired, the power controller 25 is turned on or off to increase or decrease the heat of the furnace, tending to retrol circuit in detail. I have not felt it necessary 5 turn the departed temperature to its desired to duplicate these details in other figures which illustrate modifications of a portion only of the complete circuit of Fig. 1.

- Referring now in particular in Fig. 1, I indicate at l a phase sensitive A.-C, bridge having fixed resistor arms 2, 3 and I. The fourth arm 5 of the bridge I is a resistance element located in an electric furnace 8 and sensitive to the temvalue. The actual temperature is continuously indicated on the scale ii and recorded on the chart l2.

The motor 9 in addition to positioning the con- 0 tact arm 8 simultaneously positions a contact arm 21 over a slide-wire 28. At 29 I indicate a contact arm adapted to be manuall positioned along a slide-wire 30 for establishing the temperature standard or control point to which the control works, i. e. the temperature value which is desirably to be maintained at the sensitive arm 5 The elements 21, 28, 29 and 30 comprise what I term a balanceable control network including 7 the joining conductors 3|, 32. This circuit is supplied with A.-C. power through the secondary winding 33 of a transformer 34. Contact arm 21 is positioned relative to the slide wire 28 by the motor 9 representative of actual temperature to which the resistance arm 5 is sensitive. The contact arm 23 is positioned relative to slide wire 39 manually to a point representing the desired temperature. The control circuit including these elements then establishes a signal 30 across the terminals 35, 36 oi reversible phase and having a magnitude proportional to the unbalance of the control circuit.

The operation is as follows: If temperature within the furnace deviates from the desired value, then the resistance oil-the bridge arm 5 changes, causing an unbalance of the'bridge l in one direction or the other dependent upon whether the actual temperature is above or below the desired temperature. The phase and magnitude of the A.-C. output of the bridge I follows the sense and amount of unbalance of the bridge and is appliedto the amplifier l6 for control of the motor 9. The motor 9 rotates in predetermined direction and amount determined by the phase and amount of unbalance of the bridge i and positions the contact 8 along the slide-wire l in proper direction to rebalance the bridge. The amount of movement of the contact 8' over the slide-wire I to bring about such a rebalancing is representative of the deviation of the actual temperature from the desired temperaure, and herefore the indicator l continuously shows on the index I l and chart i2 the actual temperature of the furnace.

At the same time the motor 9 has positioned the contact 21 along the slide wire 28 in consonance with the departure of the temperature from the desired temperature. With such movement of the contact arm 21 the control bridge including the elements 2?, 28, 29, 30, 3|, and 32 becomes unbalanced and an electrical value e0 is established across the terminals 35, 36 representative of the direction and extent of such unbalance. Such signal is effective to actuate the relay 26 controlling the resistor i3 through the agency of power controller 25 in a direction to return the temperature of the furnace toward the desired standard.

As the temperature (due to a change in heating of the resistor 13) returns toward desired value the consequent temperature change at the resistance unbalances the bridge I and causes the motor 9 to position the contacts 3 and 21 inproper direction and amount until the measuring bridge I and the control bridge are again balanced. It is appreciated, of course, that this action may be more or less continuous, i. e. before the temperatm'e returns completely to its desired standard there may be other influences acting upon the furnace to prevent or to accelerate the return of the temperature to the desired value. In other words, the measuring circuit is continuously indicating the instantaneous temperature of the furnace and the control circuit is continuously regulating the power controller 25 to maintain the temperature at the desired value or standard. I will now describe the operation of the apparatus within the dotted enclosure 26 which I term an electronic relay. Such apparatus is under the control of the signal e0 for regulating the power controller 25.

I have indicated as 6c the unbalance potential which may exist between the contact arms 21 and 29 of the control bridge and which unbalance voltage may be of a phase and magnitude depending upon the direction and extent of unbalance. In describing the operation I am considering that 60 60. In other words, for the present I consider that the unbalance ee'is applied directly to the terminals 35, 3'.

The magnitude of the control impulse 6c is proportional to the deviation of the measured quantity from the control setting. The sense of the unbalance depends on whether the deviation is below or above the control setting, and determines the phase or polarity of the control impulse. This impulse is amplified or applied directly to the controller. The direction in which the controller operates to I restore the measured quantity 'to standard is determined by the phase or polarity of the unbalance of the control bridge. A phase. discriminatory device 31 is shown as a. single envelope tube containing triodes 34 and 39 of which 33 is a rectifier whose function is to supply a reference D.-C. voltage E: for 33. The voltage e0 at terminals 35, 36 is impressed between the grid 40 and the cathode ll of triode 38 establishing a D.-C. voltage E1 to be compared to E2.

When the control bridge is in balance (e=0), i. e. when temperature (as represented by the relative position of 21, 28) is at the desired value (as represented by the relative position of 29, 30) triode 38 conducts apredetermined amount of current resulting in a voltage drop E1. An adjustable contact 42 is manually positioned along a resistance 43 until E2 is slightly less than E1. The voltage difference between E1 and E2 is then applied to the grid 44 of a gas-filled tube 45, such as a Thyratron, through a resistor 46 and a capacitor 41. The negative voltage thus applied to the grid of the thyratron is just large enough to prevent 45 from firing.

Inasmuch as the relay 26 being described provides an on-off control of the heating element l3 it will be appreciated that I desire to fire tube 45 and energize the mechanical relay 49 for the power controller 25 only when the temperature tends to fall below the desired standard value.

When the temperature increases above standard or desired value the tube 45 does not fire or conduct. The unbalance voltage so on an increase in temperature above the desired value, is in phase with the plate voltage of triode 38 and gives an increase in plate current over balance condition. This results in an increased voltage drop E1 across the resistor 49.

When the temperature decreases from standard the control bridge becomes unbalanced and a voltage e0 available across the terminals 35, 39 is supplied to the grid 49 of discriminator triode 38. Such voltage is out of phase with the plate voltage of 38 so that the plate current is reduced and E1 becomes less than at balance. This reduces the negative voltage applied to the grid of thyratron 45 which fires, completing the circuit to energize relay 49.

Thus the relay 26 is sensitive to the phase of the voltage ea or to the direction of unbalance of the control bridge, which is the "same as saying that the relay 26 is sensitive and responsive to a departure of actual temperature above or below the desired value of temperature. The system so far described comprises an on-oif control. When temperature decreases below the desired value then the thyratron 45 is fired, the relay 49 is energized and the power controller 25 is so activated as to increase the heat applied to the furnace 3 so that the actual temperature eifective at 5 will be raised toward the desired value. If the temperature increases above the desired value the. control bridgebecom'es unbalanced in opposite sense, applying a voltage at of opposite phase to the relay circuit 26, thereby shutting off the thyratron 45 to shut off the heat applied to the furnace 3.

The "control point as it is sometimes termed in this art is the desired or "standard" temperature value indicated on a scale 8| by hand adiustment of the contact pointer I! along the slide -wire resistance ll.

It will be appreciated that in the operation of a furnace there may be a sudden temporary drop in temperature such as would be caused by the opening and shutting of a door in the furnace wall. As such a temperature change is momentary only and is not indicative of atrend, it is desired that the control system disregard all changes in temperature whose duration is less than a predetermined time interval. I have, therefore, arranged the circuit so that such a temperature drop of a short duration will be ineffective to fire the thyratron II. The tendency of course will be to fire the tube but the voltage across thecondenser 41 cannot change as fast as does the voltage difference between E1 and E2 due to the fact that the condenser 41 holds its charge and tends to maintain its voltage in known manner. The rate at which the voltage across condenser 41 changes is determined by the size of 41 and by that portion II of a resistance 52 between the contact arm 53 and the contact arm 42. I thus designate the contact 53 as a "time delay adjustment" and provide that manually the contact 53 may be moved along the resistor 52 to adiustthe time duration of a decrease in temperature which is ineflective to fire the tube 45. The basic values of the capacitor 41 and the resistor 52 give a ready possibility of a to 1 range in time delay adjustment. For example, it is quite commercially possible that temperature decreases of a duration of 2, 5. 10 or possibly greater number of se ands will be considered as being temporary decreases only and will be ineffective to fire the tube 45. As previously mentioned, these time intervals are adjustable manually by moving the contact 53 along the resistance 52 and thus varying the value of the included resistance 5|.

I have previously mentioned the term dead ban of a controller or controlling instrument. In Fig. l I indicate a resistor 46 in the grid circuit of the thyratron 45. If the value of resistor 46 is large I get substantially no dead band, but I can introduce a dead band or increase it in value by decreasing the value of-the resistor 46. This may readily be-accomplished by substitution of fixed resistors and usually it is not necessary to change the value of the dead band for any given measuring controller after it is first installed=and adjusted to the operating conditions desired in the furnace. In other words, in many instances it is not necessary or desirable to have the dead band adjustment readily changeable manually where it could be changed by unauthorized persons.

I willnow describe the particular feature of my present invention wherein the proportion of on-time" to "off-time may be varied automatically to include in a basic onoflf" control system certain advantageous features of the "modulating" type of control system I accomplish this by modifying the control bridge unbalance signal 8c by a modulating signal em to the end that the signal e0 applied to the terminals 35, I8 is equal to en plus or minus em.

Basically the system consists of introducing a periodically cycling signal 8m in series or superimposed with the signal 8c obtained from the deviation of a controlled variable from standard. The resultant signal Co is applied either directly orthrough the medium of an amplifier or electronic relay to operate an on-oif controller, such as an electric contactor. solenoid valve. or the like The superimposed signal eis the unbalance voltage of a modulating bridge including an ad- I justable resistance element Sp and the secondary windings 54, 55 of a movable coretransformer 56. The movable core transformer 55 has an A.-C. energized primary winding 51 variably coupled to the secondaries S4, 55 by means of a movable magnetic core member 58, which is slowly reciprocatcd back and forth across a central coupling relation with the windings 51, 54 and 55.

For cyclically positioning the core II I provide linkage 59 having a roller arm 60 riding an eccentric disc or cam ii driven synchronously by a motor 62. The shape of the cam ii is relatively immaterial and may be provided with lobes to allow more than one oscillation per revolution if desired. Furthermore, the shape of the cam 6| may be such as to introduce linear or nonlinear reciprocation of the element 58 between the windings it serves to couple inductively.

Joining the elements Sp. 54 and 55 into a bridge network are outer conductors GI, 64 and a conjugate conductor 65. One end of the conductor 65 joins the mid-point of the bucking secondaries 54, 55 and the other end forms a movable contact arm engageable with the resistance Sp. Interposed in the conductor 55 is a resistance Sb across which the unbalance voltage em is taken in variable amount manually adjustable.

The magnetic core 58 is synchronously reciprocated at a slow frequency by the motor 62. Preferably this is about one complete cycle per minute although the minimum frequency should be several times as great as the natural frequency of hunting of the control system. The core is symmetrical about the center of the assembly of windings 54, 55, 51, so that when the position adjustment Sp is at fifty percent, or mid-point, the unbalance voltage em will build up to maximum and decay to zero first in phase and then out of phase with the supply, voltage, which latter is the same as the plate voltage of the relay tube 38.

Under this condition and with the control bridge 21, 28, 29, 30, 3|, 32 perfectly balanced, i. e. e=0, the relay 49 will be on exactly half the time. A deviation in the measured variable (temperature) from the standard desired will produce a control voltage 6c alternately in phase and out of phase with the superimposed periodic signal em. As a result, the ratio of duration of the in phase to the out" of phase signal is modified by the magnitude and phase of ca. Hence if the controlled variable (temperature) is down-from standard the on period of the relay 4! will increase in proportion to the magnitude of the deviation.

Fig. 2 depicts a. typical voltage envelope for the signal em for one complete cycle of the cam 6|, 1. e. for one complete reciprocation cycle of the core 58 coupling the energized primary winding 51 with the bucking secondary windings 54, 55. Assume that the core 58 is at exactly the inductive midpoint of the windings corresponding to A (Fig. 2). The unbalance voltage em=0 in phase and magnitude. As the core moves away from neutral (A toward B in Fig. 2) a sixty cycle A.-C. voltage em is developed, growing at a uniform rate from zero at A to a maximum at B, and of one phase which I designate as in-phase. As the core reverses its direction of motion and moves asses back toward neutral position the in-phase voltage are decays to zero (B to C). It will be appreciated that the curves drawn within the envelope A-B-C 'of Fig. 2 are representative only and are a part only of the eighteen hundred alternations occurring in the one-half minute hali'cycle oi cam'll.

As the core 58 passes the neutral point the signal em passes through zero in magnitude (C) and reverses in phase producing an out of phasesignal of magnitude growing along C-D and then decaying along D--E to zero. While actual values depend upon difl'erent circumstances I may say that the maximum value of the signal 8m will be about two volts while the eilective value of the signal (the R. M. S.) will usually be about one volt under the conditions of S set at midpoint and 8s set at Max.

As clearly shown in Fig. 1 the modulating signal cm is superimposed on the control signal 8c to produce the final signal so which is applied to terminals 35, 36. The two signals are applied in series so that eo=eciem. Refer now to Fig. 3. Herein I show in somewhat schematic manner the production of the final signal so for one complete time cycle of cam' 61. Although of alternating current I have assumed that the unbalance volt-' age Co has a uniform effective value of X above zero value base line. The full cycle of growth and decay in 'valu'e'of cm is shown in solid line;

The resultant voltage signal 'eo=ec:em is shown dotted. For the examples given the ratio of ontime to ofl-time"-of heater i3 is about .43.

As previously mentioned the voltage 80 at terminals 35, 36 is impressed between the grid 40 and cathode ll of triode'38, whose plate voltage is of supply phase. The relation of the two voltages as'to phase and magnitude controls the D.-C. voltage E1 which is to be compared to E2. When the signal voltage e is out of phase with the plate voltage oi triode 38, the plate current is reduced and E1 becomes less in comparison with E2. This reduces the negative voltage applied to the grid of thyratronfi, completing the circuit to energize relay 49 and turning on" the resistor (heat) l3. Thus the out-of-phase time of signal e.. is the "on" time of supplying heat to the furnace 6. 0bviously the arrangement could be reversed if desired.

The proportional band" or "control band, is that temperaturevariat ion across the control standard which requires that the full heating capacity of the resistor i3's ha'll be on" one hundred percent of the time, or shall be oiI one hundred percent of the time. In other words, if the range"'is 1Q00 F. and the control point" is 500 F. a departure of temperature of Y" F. to

' a value of 500+Y, or to a value of 500-Y will dictate that the' heater' should be oil one llllIl-' dred percent 'of the time or "on" one hundred percent of the time respectively; on the assumption that the constants and characteristics of the system are such that when temperature is as the "standard the heater is on fifty percent of the time and otTiifty percent. At or beyond '(500+Y) F. nothing further can be done to lower the temperature than to completely turn off the heater.- If the actual temperature is between 500 and 500+Y, then the heater should be off most ,of/the time, but should be "on a percentage of the time, decreasing'as temperature departs from 500 toward 500+ Y. At

or below (500- Y) "F. nothing further can be done to raise the temperature than to have the heater turned on one hundred percent of the time. As temperature rises and approaches 500 F. the heater should be 08 an increasing percentage of the time. The width of the control band is 2Y F.

I have previously shown that as the value of signal 8e approaches the R. M. B. of signal Cm (in phase) the heater "oi!" time increases-until it is oil one hundred percent oi the time, when to equals the eiiectiveem; and (out-of-phase) the heater "on" time increases to a full "on" value.

Assume that the R. M. S. of 8m is one volt. Reference to Fig. 1 will show that I preferably supply the control bridge 28, II. Ii, 32 from the transformer 34 with a voltage of II. Greatest possible unbalance of the control bridge, produced by maximum discrepancy between actual and desired temperature values (1000' F.) would give a value to Co of 50 volts. Ii 8m=1 volt, then Y F. requires that 6e=1 volt (to be "on" or "011") or 2% of 50 volts. Y then is 2% of 1000' F. or 20 F. and 2Y=40 r2, which is the control band"- for the conditions stated. The relation is substantially linear over the entire range so that the control band equals :20 F. on either side of a standard value or control point at various places along the range.

The maximum value Z oi 8m may be varied by manually moving the contact along resistance St to pick oil a desired portion of the unbalance in conductor 65. The magnitude of the "proportiom ing band" depends on the maximum value of Gin and so can be adjusted simply by means oi the 8s potentiometer.

The ratio of the "in-phase" to the out-ofphase" signal an (v./w. in. Fig. 3) may be readily and accurately adjusted by movingthe contact 65 along the resistance Sp.

In Fig: 2 I have shown the boundary of the voltage envelope as comprising straight lines, a

result which would be produced if the cam I is shaped to have a uniform rise and fall, thus providing a linear relation between movement and voltage. Under the same conditions the curves of Fig. 3 are straight lines. It will be appreciated that if the cam I is an oil-center disc or eccentric, the relation will be sinusoidal and the boundary of the envelope of Fig. 2 as well as the em and Co curves of Fig. 3 will emulate wave forms. I indicate here no distinct preference in the shape of cam I but believe that this may be In Fig. 4 I show a modification of a part only of the complete circuit of Fig, 1. Inasmuch as this particular modification encompasses a variation only in the method of establishing a modulating signal em it does not appear necessary to duplicate the measuring and controlling circuits of Fig. 1'. It appears suiiicient to describe how I propose to apply to the terminals 35, 36 a signal e. which is a composite of the signals 8c and 8m where the latter is cyclically produced in a manner somewhat diiIerent than is shown in Fig. 1.

In this modification the reciprocating magnetic core I is replaced by a resistance bridge modulator comprising a 360 slide-wire 61 tapped at two points 180 apart. The contact arm 66 is driven by a synchronous motor 82 and moves at a uniform rate along the slide-wire 61 alternately unbalancing the modulating bridge 68 in one direction or the other. The unbalance voltage 8111 provides an envelope having the characteristics of Figs. 2 and 3 with one complete cycle per revolution of the contact 66 over the slide-wire II.

This modification has certain advantages and certain disadvantages over that of Fig. '1. It can be made into a very compact unit which can be located at will. On the other hand, it has the disadvantage of a sliding open contact as compared to the inductive coupling of Fig. 1. If the slide-wire 61 is accurately wound for linearity, the timing of the device will be a very accurate function of the position of the adjusting slide wire Sb. This may be used to accurately control the average rate of change of some controlled variable as illustrated in Fig. 5, which will now be explained.

Certain elements of Fig. 5 are the same as those of Fig. 4 and bear the same reference numerals. A modulator bridge 68 includes the potentiometer Sm having a continuous slide-wire 61 tapped at two points 180 apart and a contact arm 66 synchronously rotated around the slide-wire 61 through the agency of a motor 62. The adjustable potentiometer Sp is provided with an index 59 which may be graduated in terms of rate." As previously explained, the signal 8m produced by the modulator bridge 68 fits an envelope such as is shown in Fig. 2 of linear growth and decay of voltage first "forward-phase and then re verse-phase." When the potentiometer Sp is at neutral the ratio of forward-phase to reversephase is .50. Movement of the adjustment Sp will vary the ratio of fforward-phase to reverse-phase signal em.

The signal em is applied to an electronic relay.

26A similar .to the relay. of Fig. 1, but provided with a normally open contact 49A and a normally closed contact 498. The contacts 49A, 49B are connected through a two-position switch 10 to control direction of rotation of a capacitor-run motor 1| having windings l2 and 13, as well as a capacitor 14. A common terminal of the windings 12, I3 is joined by a conductor in series through limit switches I8, 11 to one side of the power source 18.

The control bridge producing theunbal'ance signal 6c includes the control potentiometer Sc adjusted by the motor 9 along with the measuring potentiometer I, 8. The standard" potentiometer 55 includes the resistance 3|! and a contact arm 29, the latter adapted to be positioned by the motor H through the agency of a friction drive 19 in such manner that the contact arm 29 may be manually moved through the friction drive 19 relative to the motor Ii. The arm 28 has a proper extension adapted to engage the limit switches 16, 11 which are individually mounted in an adjustable manner upon a track 80 which encircles the slide-wire 30. The arrangement is such that the limit switches 15, 11 may be separately, manually positioned along the track 80 to define the limits of travel of the contact arm 29 along the slide-wire 30. Due to the friction drive 19 the motor Ii may cause the arm 29 to engage either the limit switches 16 or I! and thereafter to not stall or damage the motor I l. The arrangement described provides a means for smoothly and uniformly varying the stand ard" to which the control system is to operate. This is accomplished by a smooth, continuous movement of the arm 29 along the slide-wire 30 12 whereby the value 8' is continuously varied and at a "rate" dependent upon the proportion of the time that motor Ii is operating in one direction, 1. e. depending upon the signal Cm. 5 Assume that Sp is set at its neutral position for the bridge II. The signal cm is alternately forward-phase and reverse-phase with respect to the voltage supplied to the relay 28A and the signals are of equal magnitude. The contact "A will be closed one-half of the time and the contact 498 will be closed the other half of the time. Therefore, the motor II will rotate in one direction onehalf of the time and in the other directionthe other half of the time, and the net result will be that the travel of the arm I! in either direction will cancel out and give the effect of no movement or change in standard. It is obvious that the speed of rotation of arm I must be adJustable for various systems. If (considering an extreme condition) the forward-phase signal Cm were of several minutes duration (and the reverse-phase si nal cm of equal duration) the arm 2! would be driven to its limit of travel in one direction or the other with great disturbance in the value cc. Proper forward-phase and reverse-phase time duration of the signal Cu is probably in the nature of seconds to minimize the extent of travel of the arm 29 over the slide-wire II. The net result de- 7 sired is that the dictated position of the elements 30 29, Ill (representative of the desired "standard) will be unchanged so long as Sp is at the neutral of the modulator bridge ll. So far, the end result, insofar as the signal 80 applied to the terminals 35, 38 is concerned, is the same as described in connection with Fig. 4. It is true that the arrangement is somewhat different in that the electronic relay 26A controls the motor H.

Movement of the potentiometer Sp to one side or the other of neutral position varies the ratio of forward-phase to reverse-phase signals 8m with consequent net effect of the motor H moving the arm 29 continuously in one direction along the slide-wire 30 and at a speed'dependent upon the ratio of forward-phase to reverse-phase signals 5 6m.

As the value S8 continues to change in the same direction it influences the signal 8c and the control system causes the actual value of temperature as represented by S; to continuously follow the uniformly varying standards." Referring now to Fig. 6, I indicate that a desirable temperature may be maintained within the furnace l of uniform gradation between some arbitrary value of 300 F. and a second value of 900 time as indicated in Fig. 6-as four units of time. The slope of the curve MN is the rate of change in temperature and such slope may be varied by the adjustment Sp which varies the ratio of 80 forward-phase to reverse-phase signal em.

In many processes, as for example in the heat treatment of metals, it is desired to vary the temperature within the furnace from one value to another at a uniform rate over a given period of time. Through the features of my invention just described this is readily possible.

If the complete range of the controlling instrument is for example 0-1000 F. and that is represented by the full extent 01s., then the position of the limit switches IO, 11 positionable along the track II will represent any selected values M and N for maximum value of the standard" to be maintained. Adjustment of the limit switches determines the temperature band MN while adjustment of Sp determines the F. uniformly changing over a given period of,

As shown in Fig. 5. the contact as is closed,

which completes a circuit from the power source 19 directly through the motor winding I3 to the power source and through the capacitor 14 in series with the winding 12 across the power source. The motor thus moves in one direction of rotation. If the contact 49A is closed, and the contact 493 is open, then the winding I2 is directly across the power source, while th winding II is in series with the capacitor I4 across the power source and motor rotation is in reverse direction.

If the adjustment Sp is moved in the opposite direction from its neutral, then the ratio between forward-phase and reverse-phase signal em passes the unity mark and the predominance of motion of thearm 29 over the slide-wire 39 is in a decreasing direction, such as along the graph N of Fig. 6.

In order however to obtain the temperature gradation MNO as shown in Fig. 6, it is necessary that the slope of the line NO be the same as the slope oi the line MN. This dictates that Sp would have to be set at exactly the same distance on the reverse side of its neutral as it was originally set. A more convenient way to obtain the graph MNO is to utilize the hand switch HI when value N is reached.

Assume that Sp has been adjusted to produce the increase in temperature curve MN and when the value of temperature N is reached it is desired to decrease the temperature of the furnace at exactly the same rate. When N (in terms'of time) is reached the switch Ill is thrown manually to its reverse position and the predominance of forward-phase over reverse-phase (or vice versa) signals cm are thereafter applied to the windings of motor Ii producing reverse rotation, and consequently gradually moving the arm 29 back along its previous path over the slide-wire 39 with the result'that the standard'Ss is reduced at the same rate NO that it was increased along MN.

It will thus be apparent that in Fig. 5 I have disclosed a ready means for controlling in accordance with a preselected rate of change in the variable, and to maintain the variable at such a preselected rate of change in either an increasing or a decreasing manner. I have provided a ready means for adjustably varying both ordinates of the plot of the variable against time, namely, an adjustment of the extreme values of the variable and of the slope of the increase or decrease. I have further provided ready means for reversing the trend of control at the previous rate or for thereafter manually varying the rate as desired.

In certain processes it is desirable to have a dwell, such as NP, to produce a temperature curve MNPQ for example. For example, in the heat treatment of certain metals it is desired to gradually raise the temperature to a preselected maximum, to then maintain this maximum over a certain period of time and to then gradually reduce the temperature to a preselected minimum. This is shown in Fig. 6 as MNPQ. With the features of Fig. 5 this may readily be accomplished.

I have previously mentioned that the graph MN is accomplished by net movement of the motor H in one direction, thereby moving the contact arm 29 for example from the limit switch 14." 19 to the limit switch 11. When either limit switch is engaged, the conductor 19 is disconnected, from the power source 19. The motor thus stops and the "standard" 8- remains unvarying, producing the dwell NP until the stand-. ard is again changed manually, or started manstandard of 900 F. the operator reverses the switch 19 to dictate reverse rotation of the motor II and then momentarily depresses the push button 9| which by-passes the limit switches 19, I1 and energizes the motor H for a suiilclent length of time to back the contact arm 29 away from the limit switch it had previously engaged thus allowing the limit switch to close circuit. Thereafter the push button 8| may be released and current for the mot-or H is available through the limit switches 19, ll as previously, and with the motor ll rotating in reverse direction to produce the graph PQ. I

It is frequently desired to produce a controlled temperature graph MNO completely automatically without the necessity of manual manipulation to reverse motor rotation by means of the switch 10 when N is reached. In Fig. 7 I show a possibility of automatically carrying out the temperature cycle MNO. Herein the manually actuated switch 19 has been replaced by a magnetic relay 92 having a winding 99. is shown in Fig. "I for a condition wherein the motor II is moving the contact arm 29 in clockclockwise and eventually engagin limit switch ll, closing the same. This completes a'circuit I8, 84, 99, 9|, ll, 92, 98, 94, 19 energizing relay 93 which opens contacts 96 and 99 and closes contacts 99, 94 and 95. Contacts 99 and 94 reverse direction of rotation of the motor "II relative to the contacts 49A and 493. Contact 99 latches in the relay coil 83 through closed limit switch 15. I

The energization of winding 99 immediately starts motor H to moving the contact arm 29 in net counterclockwise direction along the path NO (Fig. 6) which continues even after arm 29 has backed away from limit switch 11, opening the I same.

I The rotation will continue iri counterclockwise direction until limit switch I9 is reached and opened deenergizing 93. which immediately initiates a net reversal of the motor with the 5 resultthat if left alone the system would continuously cycle over a path MNO. To stop such .a cycle at point 0, or in fact at any point along the line MNO, it is only necessary to open switch 99,

which kills motor II and leaves the arm 29 at a value of 8.5, which thereafter dominates the controlle'r.

When the arm 29 is stopped (by opening switch 94) at limit switch 16 or any location toward ,ll,

. a closure of switch will cause the arm 29 to ,for the desired length of the dwell.

curve, then it is only necessary to close the-switch The circuit 88. If it is desiredfrom the dwell to start a decreasing temperature rate, then after the switch 88 is closed it is possible to momentarily depress the switch 86 long enough to energize relay 8!, which immediately locks in through the limit switch 18 and results in counterclockwise movement of the arm 28.

In Fig. 8 I show a modification of Fig. '1 wherein the length of dwell NP may be preselected and the path MNPQ automatically attained. The principal differences consist in the addtion of a timer 9! driven by a synchronous timing motor 88 and a change in the nature of the limit switch 11A. Herein, as in Fig. 7, the circuit is shown with the motor TI positioning the arm 28 in a clockwise direction somewhere intermediate the limit switches 16, 11A. The circuit issimilar to that of Fig. 7 except that intermediate the switch 88 and the conductor 88 the power line is connected by conductors I02, I08 through a normally closed finger oi the limit switch "A.

When the arm 20 engages the limit switch "A the circuit is closed (as in Fig. 7) between the conductors 92 and SI for energizing the reversing relay 82. However, at the same time-the limit switch "A breaks circuit I02-I08 and prevents the relay 82- from being energized. Simultaneously, however, circuit is closed between the conductors I08 and I08 which energizes the synchronous motor 98 through the conductor I8I to the main switch 88. The. motor 88 synchronously drives the timer 81, which has provision for man ually adjusting the time length of rotation before a movable contact I05 engages a, stationary contact I 08 closing circuit between the conductors I 02, I03. This preselected increment of time is thedesired length of dwell NP.

When contact I05 engages contact I08 power is available through a circuit 88, 02, TIA, 8I to energize winding 88, which then'locks itself in through 95, I00, 18, 92, "2,105, I08, 11A, I84, I03. As arm 29 backs away from limit switch "A the motor 98 is deeneergized, as is contact I08. In other words, the. motor 98 operates, and the contacts I06, I05 are operative, only when the arm 29 is at its limit of travel depressing the limit switch 11A to the alternate position of that shown in Fig. 8.

As soon as the motor 1| begins to move arm 29 in counterclockwise direction it releaseslimit switch TIA. which returns to the position shown in Fig. 8. Motor I I continues to move arm 28 in counterclockwise rotation until the latter engages limit switch 18 and opens the same, thus stopping the motor bydeenergizing relay winding 88, The system has produced the time curve MNPQ. The length of NP is adjustable on the timer 8!.

The rate slope MN and PQ are the same, as adjusted on Sp. As previously mentioned, the decay slope may be varied (as PR) by manually shifting the rate adjustment 8 when time location P is reached.

Following the cycle MNPQ the relay 88 isdeenergized and the .complete cycle would immediately begin to repeat unless the operator at that time opens the main switch 88. This is in any event advisable because many variables enter into the desirability oi immediately repeating, the complete cycle or predicting how long the time dwell should be had at the lowermost tempera-- ture. Such a time dwell depends, of course, upon certain things such as the, time necessary to unload or reload a furnace'or process...

In Figs. 9, .10 and 11 I show a modification of the arrangement described in connection with 16 Figs. 5, 8, 'l and 8. The arrangement is such as to provide the possibility of automatically performing a complete cycle MNPR (Fig. 6) or of selectively utilizing hand control. Fig. 9 shows the complete circuit intermediate the measuring slide-wire l, 8 and the output signal supplied to terminals 88, 88. Fig. includes certain elements of Fig. 9 shown in across-the-line" manner for ready circuit tracing. Fig. 11 depicts the face at a panel board which contains the various hand adjustable elements of Figs. 9 and 10. In the following description reference should be made to Figs. 9, l0 and ii.

I show the modulator bridge 88 (producing the signal em) as having a rate oi increase" potentiometer RI and a rate of decrease Potentiometer RD in addition to the synchronously driven potentiometer Be. The potentiometers RI and RD are selectively included in the bridge circuit 88 by a contact arm I81 ensuing either contact I or contact D in the alternate positions of a relay RA. The potentiometers RI and RD respectively refer to and preset the rate or increase or rate of decrease of the variable, namely, the slope of the line MN and oi the line PR (Fig. 6). These potentiometers RI and RD are hand adjusted with reference to graduated scales bearing the same 1titles on the suggested panel board of Fig. 1

Th premise used in describing Figs. 9, 10 and 11 is that the control is operating in an increasing temperature direction somewheres along the line MN between M and N. Therefore the switch arm I0! is contacting at the point I with relay RA deenergized. This predict that the modulator bridge 88 includes the elements RI and 8m but does not include the element RD. The output of bridge 88, namely, signal 8m, produces a forward-phase" and a reverse-phase" signal Fig. 2.

The signal 8m is applied to an electronic relay ERI having a normally closed contact I08, which is connected directly across the conductors I88, III of the motor 'II. The contact I88 parallels the motor capacitor 18 and a resistance III, for shorting out the same (when closed) and plugging the motor. Direction of rotation of the motor II is either clockwise or counterclockwise, dependent upon whether the contact arm III of the relay RA engages CCW or CW. As shown, the'motor is connected for CCW rotation .during the portion of the time that contact I88 is open. As previously explained, the proportion of "open to closed" time of the contact I88 isrelated to the relation of- "forward-phase" to "reversephase" of the signal Cm. Inasmuch as the motor 1i rotates only when the contact I88 is open, and then only in either CW or CCW direction, the to motor operation diners somewhat from that; previously described. Referring to Fig. 12, it will be observed that the progression of slope MN is accomplished by the motor of Fig. 5 as a dinerence between the CW and CCW alternate move- 05 ments of the motor. In other words, as previously explained, the motor rotates a small amount CW and then a small amount CCW alternately, and the progression is by the difference of saidmovements, both as to, slope (rate) of the line and the direction of preponderance determines the direction or movement. On the other hand the motor of Fig. 9

' moves only-inn single direction and inches along by the duration of the 8m signals which "open contact I88.

similar that shown as a voltage envelope in Those signals which "close'! cone tact I88 short the capacitor 14 and plug the motor to a stop. Direction of rotation is controlled entirely by the position of the contact arm II2 engaging either CCW or CW. In the example being described in connection with Fig. 9 the motor 1| is rotating CCW at a rate determined by the position of RI and'upwardly along the increase line MN.

The circuit for CCW motor rotation' may be traced; I8, I25, normally closed contact I28,I2I, (winding 13) (winding 12, resistor III, capacitor 14), I88, contact CCW, II2, I28, I2I, I22; I23, I24, and power source 18. The circuit for CW rotation is as follows:"I8, I25, I28, I21, (13, I4, III) (12), III), CW, H2, H3, H4, I22, I23,-I24 and I8.

Turning now to the control bridge originating the signal ea, it will be observed that the bridge includes the control potentiometer Sc positioned by the balancing motor 9 along with the measuring balancing potentiometer I', 8 so that the value of Se is representative of the actual temperature being measured. The bridge also includes the standard potentiometer S5 wherein the position of the contact arm 28 along the slide-wire 30 is representative of the desired standard temperature to be maintained at any instance, up to a maximum value preset by hand adjustment of potentiometer RM. The signal 8c is the unbalance of the bridge including potentiometers Sc and Si, namely, a signal of phase and magnitude showing the discrepancy, if any, between the actual temperature (value of Se) and the desired or standard temperature (the value of 85) where the latter is being continually changed according to the program desired.

The output of that portion of the bridge including the potentiometers RM and S; is applied to an electronic relay ER2 having an output contact I32 normally open; and close circuited only when the standard value S8 reaches the preselected maximum standard RM.

The control bridge includes a hand set single control point value RS whose contact arm is con-' nected to a terminal I3I and whose function will later be described.

The drawingof Fig. 9 includes a timer T having a contact TC which is normally open circuited and which becomes closed after the timer T has,

rotated for a preselected length of time. When the timer motor is thereafter deenergized a spring return opens the contact TC and resets the time element so that a preselected interval of time of operation of the motor T must be had before TC is again closed. I will now describe the general operation of the circuit.

Assume that the actual temperature is intermediate the points M-and' N, increasing at a preselected rate along MN determined 'by the setting of RI, driving the motor II in proper direction to position the contact arm 29- of SS in proper direction and speed so that the output signal 6c produces a control which advances temperature along the line MN.

When a predetermined maximum temperature N is reached as evidenced by the value S=RM, and the signal applied to EH2 becoming zero, then contact I32 closes. This completes circuit 18, I31, T, I32, I39, II8,'I.I5, IIS, I34, I35, I38, 18, starting the timer motor T. It also completes the circuit 'I8,RB', I38, I32, I39, H8, H5, H6; I34, I35, I38, I8, to energize the relay ,RB. Energization of the relay RB opens contact I20I2I', de-

18 Stopping of the motor II, which leaves 85 at a maximum value preset at RM, means that the actual temperature has reached the plateau NP (Fig. 6) and such value will be maintained so long as S5 is not moved. This value S; will not be varied until the timing motor T has rotated its preselected length of time and engages to close contact TC at a time P (Fig. 6). In other words, the timer is set for an interval of time corresponding to the time length of the desired dwell .NP. When such interval of time has been expended, then contact TC is closed. During this time interval the signal suppl ed to the relay ERZ remains zero and the contact I32 is held closed to complete circuits through the relay RB and the timing motor T. Energization of the relay RB also effects a closing of the contact II9-I I8 to short circuit the switch I32 and assure a holding of the circuits through the relay and the timing motor closed.

Closure of TC completes a circuit I8, I36, I35. TC, I33, RA, I8, energizing relay RA which looks itself in through a circuit I8, I36, I35, I34, 6, ill, RA, I8. At the same time energization of RA causes a movement of contact I I6 from I I5 to I II, a closure of I I3I I4, a movement of contact In from CCW to CW, and a movement of contact I01 from I to D. Movement of the contact IIE away from II5 results in a breaking of the circuits through the relay RB and the timing motor T. Contact I2I-I2fl is then completed and contact II9-I I8 broken as a result of the deenergizing of relay RB. The timing motor T resets itself and. opens the contact TC as soon as it is deenergized. The relay RA remains energized as a result of the holding circuit established by a closure of contact IIIi-II1. The movement of contact II2 to CW and the closing of contact II3I I4 results in a circuit from the motor II, through CW, H2, H3, H4, I22, I23, I24 to the power line I8 so that immediately contact 29 in direction opposite to which it had 1 previously moved and decreasing the value of energizing contact H2, and thereby stopping SS. Controlled temperature now begins to decrease along path PR or PQ at a rate determined by the setting of RD by reason of the engagement of contact III! with D during the time that the motor is connected for rotation in CW direction. It will be appreciatedthat the closing of contact I28I 2| will complete the circuit for operating the motor in the reversed direction but, in order that the motor 'Il may start in the reversed direction as soon as the timing motor opens the contact TC, the contact II3I I4 is closed to short circuit the contact I 28--I2I when the relay RA is energized.

The value is now gradually reduced as predicted by the setting RD until the arm 28 reaches a predetermined minimum value as limited by a mechanical limit. The potentiometer SS may be provided with limit switches IS, IT as previously described in connection with Fig. 5, and these may be readily connected into the electric circuit of the motor II to stop the same. However, in the present description it is only necessary to move the arm 29 until it engages a limit stop whereupon the slip clutch 19 will keep the motor "II from being damaged and will allow the controlled temperature to be maintained on a dwell at a value R until otherwise varied.

While it would be possible to predetermine the time length of the dwell at value R, this is not usually required, for after the process cycle has been accomplished it may be necessary to remove 19 the charge from the furnace and to replace it with a new charge, and thus the time interval at value B, may be of indeterminate length.

Whenever it is desired to restart the cycle MNPR it is necessary only to momentarily depress the spring closed reset push button I" which deenergizes RA and permits the contact arms I01, H2, H3 and IIS to move to their original positions. Immediately the system is in the condition shown in Fig. 9 and ready to begin a cycle along the line MNPR.

If at any time along an increasing or decreasing control line, such as MN or PR, it is desired to level off or dwell it is necessary only to throw the 3 -pole switch I28 and move the hand adiust able potentiometer RS to the value of temperature to be maintained as a single point control. Throwing the switch I28 opens contacts I23 and I 26, completely deenergizing the control circuit of motor II. Swinging the arm I 29 from the contact I30 completely disconnects the signal e; from ER2, RM, S and upon engaging the terminal I3I places the control bridge comprising Se and RS directly to produce a signal es. Thereafter in accordance with the hand set value of RS the signal 60 will depend upon discrepancy between Se and RS to maintain the actual temperature at the preselected value RS so long as may be desired. At any time thereafter the system may be returned to program control bythrowing the switch I28 into the position shown in Fig. 9.

In Fig. 13 I show a modification of Fig. 1

wherein a single modulator may serve a plurality of controllers. In view of the explanation of Fig. 1 it seems unnecessary to go into detail as to the circuit of Fig. 13.

It will be appreciated from the above detailed description that my invention has wide application and is not limited to the specific examples illustrated and explained. While I have described a single embodiment comprising the control of an electric furnace it will be appreciated that the invention is equally applicable to fuel fired furnaces, or in fact. to the control of other variables than temperature.

Certain features of my present invention, originally disclosed and claimed herein, are disclosed and claimed in my copending divisional applications S. N. 68.027, filed December 29, 1948, and S. N. 104.870 filed July 15, 1949.

What' I claim as new, and desire to secure by Letters Patent of the United States, is:

1. A furnace control system comprising a furnace; a heater for said furnace; an on and off control device for the heating medium for said heater; a temperature indicating system for said furnace including a network having an element responsive to furnace temperature adapted to unbalance the same, and rebalancing means for the network including a reversible motor for indicating the degree of unbalance and hence the temperature; a balanceable control bridge having connected, commercial frequency alternating current energized potentiometers each with a slider, one slider being connected for movement by said motor, means to adjust the other slider to designate a desired standard temperature for the furnace; phase sensitive means energized from the alternating current signal potential developed between said sliders to actuate said device to "on when the furnace temperature is below said standard and to actuate said device to "off" when the temperature is above said standard; means to produce an alternating 20 current signal potential of the said commercial frequency periodically cycling at a slow rate between aero and a maximum with alternate cycles in and out of phase with said first signal potentie]; and means to combine said signal potentials for joint control of said device.

2. A furnace control system comprising a furnace; a heater for said furnace; an on and "01! control device for the heating medium for said heater; a temperature indicating system for said furnace including a network having an element responsive to furnace temperature adapted to unbalance the same, and rebalan'cing means for the network including a reversible motor for indicating the degree of unbalance and hence the temperature; a balanceable control bridge having connected, commercial frequency alternatingcurrent energized potentiometers each with a slider, one slider being connected for movement by said motor, means to adjust the other slider to designate a desired standard temperature for the furnace; phase sensitive means energized from the alternating-current signal potential developed between said sliders to actuate said device to on when the furnace temperature is below said standard and to actuate said device to "oil" when the temperature is above said standard; means to produce an alternating current signalpotential of the said commercial frequency periodically cycling at a slow rate between zero and a maximum with alternate cycles in and out of phase with said first signal potential; means to combine said signal potentials for joint control of said device; and means to regulate the ratio of in phase to "out of phase time in said second signal.

3. The system of claim 1 in which means is provided to adjust the potential of said second signal to effect changes in the proportioning band of furnace control.

4. A furnace control system comprising a furnace; a heater for said furnace; a relay for on and "09? control of the heater; a temperature measuring system for said furnace including a network having an element responsive to furnace temperature adapted to unbalance the same and rebalancing means for the network including a reversible motor for indicating the degree of unbalance and hence the temperature; a balanceable control bridge having connected potentiometers each with a slider. a source of commercial frequency alternating current for energizing the bridge; one slider being connected for movement by said motor; means to adjust the other slider to determine a standard temperature for the furnace; a conjugate conductor connecting said sliders to provide an alternating current signal potential of reversible phase; phase sensitive means adapted to close said relay when the furnace temperature is below said standard and to open the relay when the temperature is above said standard; means to produce an alternating current signal potential of the said commercial frequency whose R. M. S. value periodically cycles at a slow rate between zero and a maximum with alternate cycles in and out of phase with said first signal potential; and means to combine said signal potentials for joint control of said phase sensitive means.

5. A control means for a system comprising -in combination; a conditioning means for said 21 control bridge having connected, commercial frequency alternating current energised potentiometers each with a slider, one slider being connected for movement by said power means. means to adjust the other slider to designate a desired standard condition for the system; phase sensitive means energised from the alternating current signal potential developed between said sliders to actuate said controller to "on" when the system condition is below said standard and to actuate said controller to "off" when the system condition is above said standard; means to produce an alternating current signal potential of said commercial frequency periodically cycling at a slow rate between zero and a maximum with alternate cycles having the commercial frequency potential in and out of phase with said first signal potential; and means to place said signal potentials in Joint control of said "on" and o controller.

6. A control means for a system comprising in combination; a conditioning means for said system; an "on" and "of!" controller for said conditioning means; a condition indicating apparatus i'orsaid system including power means responsive to system condition; a balanceable control bridge having connected, commercial frequency alternating current energized potentiometers each with a slider, one slider being connected for movement by said power means.

means to adjust the other slider to designate a desired standard condition for the system; phase sensitive means energized from the alternating current signal potentialv developed between said sliders to actuate said controller to 'on" when I the system condition is below said standard and to actuate said controller to "of!" when the system condition is above said standard: means to produce an alternating current signal potential of the said commercial frequency periodically cycling at a slow rate between zero and a maximum with alternate cycles having the commercial frequency potential in and out of phase with said first signal potential; means to place said signal potentials in joint control of said 'cn and off" controller; and means to set the ratio of "in phase" to "out of phase time in said second signal.

7. A control means for a system comprising in combination; a conditioning means for said system, an "on and "03 controller for said conditioning means, a condition indicating apparatus for said system including an element responsive to system condition and a reversible motor for indicating the condition; a balanceable control bridge includin connected potentiometers each with a slider, one slider being arranged for movement by said motor, means" to adjust the other slider to designate a desired standard condition for the system; a source of commercial frequency alternating current energizing said bridge; phase sensitive means ener-' gized from the alternating current signal potential developed between said sliders to actuate said controller when the system condition departs from said standard to return the same toward said standard; means to produce an alternating current signal of normal frequency periodically cycling in potential at a slow rate between zero and a maximum with alternate cycles having the commercial frequency of opposite phase; and means to modify said controller actuation by said second signal potential.

'8. A control means for a system comprising 22 incombination; aconditioning means for said system, an on" and "of!" controller for said parts from said standard to return the same toward said standard; means to produce an alternating current signal of normal frequency periodically cycling in potential at a slow rate between zero and a maximum with alternate cycles having the commercial frequency of opposite phase; means to modify said controller actuation by said second signal potential; and means to predetermine the rate of said modiilcation.

9. A furnace control system comprising a furnace, a heater for said furnace, an on and "of!" control device forth heating medium for said heater, a temperature indicating system for said furnace including a network having an element responsive to furnace temperature adapted to unbalance the same, and rebalancing means for the network including a'reversible motor for indicating the degree of unbalanc and hence the temperature, a balanceable control bridge having connected, commercial frequency alternating current energized potentiometers each with a slider, one slider being connected for movement by said motor, the other slider being adjustable manually to designate a desired standard temperature for the furnace, a control bridge including a transformer having an alternating current energized primary winding and a pair of like secondary windings, a potentiometer having a movable contact, means connecting said secondary windings and said potentiometer in a closed circuit, a core movable relative to said primary and secondary windings for varying their inductive relationship, means for oscillating said core through a given path, an output circuit connected to said movable contact and to a point between said secondary windings, means for combining the voltage of said output circuit with the voltage between the sliders of said first mentioned control bridge, and phase sensitive means energized by the combined voltages and operating to control said devices.

10. A furnace control system comprising a furnace, a heater for said furnace, an "on and "ofl control device for the heating medium for said heater, a temperature indicating system for said furnace including a network having an element responsive to furnace temperature adapted to unbalance the same, and rebalancing means for the network including a reversible motor for indicating the degree of unbalance and hence the temperature, a balanceable control bridge having connected, commercial frequency alternating current energized potentiometers each with a slider, one slide being connected for movement by said motor, the other slider being adjustable manually to designate a desired standard temperature for the furnace, a control bridge including a 360 slidewire, means for moving a slider over said slidewire at a uniform rate, a ,potentiometer connected at its ends to said slidewire at points 180 apart, a slider for said potenjtiometer, means for connecting said last mentioned bridge across said potentiometer to a source of alternating current, a circuit including the sliders of each of said control bridges, said sliders connected in said circuits so as to combine the output voltages of said bridges, and 5 phase sensitive means energized by the combined output voltages and operating to control said device.

11. A system for controlling a condition comprising, ln combination, means for determining said condition, a controller for said determining means, a condition indicating apparatus including power means operating in response to changes in the condition, a control bridge including potentiometers connected in the closed circuit and each having a slider, means connecting one of said sliders for movement by said power means, the other slider being adjustable manually to designate a desired condition to be maintained,

means connected across said potentiometers for said slidewire at points 180 apartra movable contact for said potentiometer, and anoutput circuit connected to said slider and said. movable contact for delivering a cycling potential.

13. The system of claim 11 in which said second control bridge includes a transformer having a primary winding and a pair of like secondary windings, a core member movable relative to said primary and secondary windings for varying their inductive relationship, means for oscillating said core member in a given path, a potentiometer connected in a closed circuit with said secondary windings, a movable contact for said potentiometer, and an output circuit connected to said movable contact and to a point between said secondary windings for delivering a cycling potential.

ANTHONY J. HORNFECK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,795,753 Bonn Mar. 10, 1931 2,220,028 Smith Oct. 29, 1940 2,246,186 Prince June 17, 1941 2,275,928 Tophan Mar. 10, 1942 2,300,537 Davis Nov. 3, 1942 2,333,393 Ryder Nov. 2, 1943 2,344,991 Lilia Mar. 28, 1944 2,359,651 Krogh Oct. 3, 1944 2,395,515 Stoller Feb. 26, 1946 2,409,136 Lilja Oct. 8, 1946 2,418,052 Side Mar. 25, 1947 

